Modern drilling techniques employ sensors downhole and broadcast this sensor information to surface using Measurement While Drilling (MWD) telemetry. One variety of MWD telemetry, Electromagnetic (EM) telemetry, sends data to the surface using low frequency radio waves generated by producing a voltage across an insulated gap sub in a drill string or assembly. In EM telemetry applications, a carrier signal is produced by applying an alternating voltage across the insulating gap or electrical isolation in the drill string. This mechanism responsible for providing the insulated gap or electrical isolation may be referred to in the art as a gap sub-assembly or gap sub. As such, the terms insulated gap, isolation sub, and gap sub may be used interchangeably herein. This EM carrier signal can be received at the surface and decoded as EM telemetry, or it can be broadcast into a formation and measured to determine EM resistivity.
An electrical isolation or gap sub must be robust and predictable, as well as, provide high mechanical strength in various drilling environments and drilling conditions. This has been difficult to obtain simultaneously in a gap sub.
An improved gap sub assembly and method for making the gap sub provide a low cost, high strength, easily maintainable and reliable isolation sub to be used as a downhole gap sub assembly.